Huge quake resonates across oceans, continents

A day after the strongest earthquake in 40 years, the effects were still being felt around the globe. The massive tsunamis created by the quake were so powerful that they spread into and across the Pacific Ocean, causing the sea to fluctuate almost a foot in San Diego and nearly 9 feet in one city on Mexico's west coast, according to the Pacific Tsunami Warning Center.

Scientists said that a sea level change probably occurred even along the New England coast, though it would have been detectable only by sensitive instruments. And they speculated that the "megathrust" quake unleashed so much energy that it could have rocked the earth off its axis a bit -- though so slightly that it would have no practical impact.

The earth has been resonating like a bell struck by a heavy mallet since two tectonic plates collided deep under the Indian Ocean, 155 miles southeast of the Indonesian island of Sumatra. The earth's crust consists of moving plates that grind against one another and in some places overlap. Pressure builds up over many years until colliding plates give way with a sudden, powerful jolt. In this case, the floor of the Indian Ocean dove under the Burma plate, driving up a 600-mile-long ledge of crust anywhere from 20 to 50 feet on the seafloor, several scientists estimated.

On the nearest land mass, the ground didn't move nearly as much -- probably rising or falling just a few feet in places, scientists said. The earthquake was far enough offshore that the surrounding land was relatively protected, according to John Ebel, director of the Weston Observatory, a geophysics research laboratory run by Boston College.

The tremors from the 9.0-magnitude earthquake have been reported by scattered observers mostly as "weak" to "moderate" shaking, with no to "very light" damage in surrounding coastal cities, according to a website run by the United States Geographical Survey, suggesting that the massive damage and tens of thousands of deaths were caused by the tsunamis, not the quake.

The sudden rise in the seafloor displaced a massive amount of water, creating the tsunami, a Japanese word derived from "tsu," meaning harbor, and "nami," meaning wave. (The term "tidal wave" is a misnomer, since tsunamis are caused by seismic disturbances, not the lunar cycle.)

The wave moves in the depths of the ocean. At the surface of the deep, open ocean, tsunamis are barely noticeable.

But when the "tsunami takes off from this place [where the earthquake occurred] in the deep ocean, it travels with the speed of a jetliner," said Klaus Jacob, senior scientist at Lamont-Doherty Earth Observatory at Columbia University. As the wave approaches the coast and the sea gets shallower, the wave slows and grows in height and eventually breaks, high over the coast.

The quake also sent shockwaves deep into the earth, and those vibrations are still being felt, and will probably continue -- providing valuable data for scientists trying to understand the composition of the deepest core of the earth.

"The [tremors] propagate around and around and they die off with time, but we'll see the waves from these main shocks for days and days," said Harley Benz, scientist in charge of the National Earthquake Information Center in Denver.

By looking at vibrations of seismic waves reflected and absorbed by layers in the interior, scientists hope to find out what the temperature is like and which parts of the earth are molten or fluid.

The massive earthquake is the largest to occur since scientists have had an extensive network of digital seismic sensors spread across the globe, and will yield far more information than smaller, more frequent events.

Ebel was among scientists who said the quake might have shifted the earth's axis ever so slightly.

"You had a section of rock 600 miles long that shifted 40 feet up. . . . That's enough to actually very slightly change the location of the spin axis -- we call it the North Pole," said Ebel.

Scientists say they will need time to go through the data collected by their sensors to see how the earthquake affected the planet. They also hope to better understand how tsunamis form.

Carolyn Johnson can be reached at cjohnson@globe.com.